Consolidate multiuser DB migrations 27–29 into a single migration step

[Copilot]

Summary

Migrations 27, 28, and 29 were added separately during multiuser branch development. Since none have shipped, consolidate them into a single atomic migration step (26→27).

Changes:

• migration_27.py: Absorbs all logic from 28 and 29 — multi-user tables, user_id columns on existing tables, client_state per-user restructuring (including old-schema data migration), and app_settings table with JWT secret generation
• migration_28.py / migration_29.py: Deleted
• sqlite_util.py: Removed imports and registrations for build_migration_28 / build_migration_29
• tests/test_sqlite_migrator.py: Replaced separate test_migration_27_* / test_migration_28_* tests with consolidated tests covering users tables, client_state migration paths (with/without existing data column), and app_settings/JWT secret

Related Issues / Discussions

QA Instructions

Run pytest tests/test_sqlite_migrator.py. All 22 tests should pass.

Merge Plan

This touches the DB migration chain. Verify no environments have already run migration 28 or 29 before merging — if they have, a gap-filling migration will be needed instead.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

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